How latex film formation and adhesion at the nanoscale correlate to performance of pressure sensitive adhesives with cellulose nanocrystals

AbstractEmulsion polymerized latex-based pressure sensitive adhesives (PSAs) are more environmentally benign because they are synthesized in water but often underperform compared to their solution polymerized counterparts. Studies have shown a simultaneous improvement in the tack, and peel and shear strength of various acrylic PSAs upon the addition of cellulose nanocrystals (CNCs). This work uses atomic force microscopy (AFM) to examine the role of CNCs in (i) the coalescence of hydrophobic 2‐ethyl hexyl acrylate/n‐butyl acrylate/methyl methacrylate (EHA/BA/MMA) latex films, and (ii) as adhesion modifiers over multiple length scales. EHA/BA/MMA. Thin films with varying solids content and CNC loading were prepared by spin coating. AFM revealed that CNCs lowered the solids content threshold for latex particle coalescence during film formation. This improved the cohesive strength of the films, which was directly reflected in the increased shear strength of the EHA/BA/MMA PSAs with increasing CNC loading. Colloidal probe AFM indicated that the nano-adhesion of thicker continuous latex films increased with CNC loading when measured over small contact areas where the effect of surface roughness was negligible. Conversely, the beneficial effects of the CNCs on macroscopic PSA tack and peel strength were outweighed by the effects of increased surface roughness with increasing CNC loading over larger surface areas. This highlights that CNCs can improve both cohesive and adhesive PSA properties, however, the effects are most pronounced when the CNCs interact favorably with the latex polymer and are uniformly dispersed throughout the adhesive film., Methods MFP-3D Atomic Force Microscope (Asylum Research – Oxford Instruments, Santa Barbara, CA, USA) Igor Pro 6.0 running Asylum Research AFM software (version 13.17)

摘要：以乳液聚合乳胶为基材的压敏胶（pressure sensitive adhesives, PSAs）因以水为合成介质而更具环境友好性，但多数情况下其性能逊于溶液聚合制备的同类胶黏剂。已有研究表明，向各类丙烯酸酯类压敏胶中添加纤维素纳米晶（cellulose nanocrystals, CNCs）可同时提升其初粘性、剥离强度与剪切强度。本研究采用原子力显微镜（atomic force microscopy, AFM）探究纤维素纳米晶的双重作用：一是调控疏水型丙烯酸异辛酯/丙烯酸正丁酯/甲基丙烯酸甲酯（2-ethylhexyl acrylate/n-butyl acrylate/methyl methacrylate, EHA/BA/MMA）乳胶膜的颗粒聚结过程，二是作为多尺度黏附改性剂发挥效用。本研究通过旋涂法制备了固含量与纤维素纳米晶添加量各不相同的EHA/BA/MMA乳胶薄膜。原子力显微镜观测结果显示，纤维素纳米晶可降低成膜过程中乳胶颗粒聚结所需的固含量阈值，进而提升薄膜的内聚强度，这一效果直接体现在随纤维素纳米晶添加量提升而增强的EHA/BA/MMA压敏胶剪切强度上。胶体探针原子力显微镜测试表明，在可忽略表面粗糙度影响的小接触面积下，较厚的连续乳胶膜的纳米级黏附力随纤维素纳米晶添加量的提升而增强。与之相反，当测试面积较大时，纤维素纳米晶添加量提升带来的表面粗糙度增大效应，抵消了其对宏观压敏胶初粘性与剥离强度的有益提升作用。该结果表明，纤维素纳米晶可同时改善压敏胶的内聚与黏附性能，但仅当纤维素纳米晶与乳胶聚合物具有良好相互作用且在胶膜中均匀分散时，其改性效果才最为显著。实验方法：采用MFP-3D型原子力显微镜（Asylum Research – Oxford Instruments，美国加利福尼亚州圣巴巴拉市），搭载Asylum Research AFM软件（版本13.17）的Igor Pro 6.0平台进行相关测试。